...
ive writen a massive essay 3 times now and frankly im kinda pissed off at this forum right now. damn you forum for not saving my stuff. 
so im gonna keep it short.
Well, let me elaborate with a tiny wall of text:
Well, as i told you: it isn't exactly "cheaper", it's just a
less bloated mainboard. Btw.: if you like durable mobos in the 75-200 range, you should check out Gigabyte. ASRock certainly isn't as bad it's reputation anymore.
CPU wise i never argued for or against a dual CPU build. 5 heavy duty users will certainly make sure a dual E5 isn't bored, but you still haven't provided an actual workload scenario, and therefore it's rather tough to recommend anything. Keep in mind your first build had a 630$ mobo + CPU combo, a dual E5 build would be around 900 - with certainly way better bandwith, cache, and performance overall - IF that's even required though, otherwise it doesn't make any sense of course.
But let's go for a single CPU setup. It depends now on the actual workload, again, and of course if you OC or not. If you won't OC, then the 4790k is a complete waste of money. You also have to add the cooling tax - no OC comes for free.
Small price / performance comparison with an alternative:
http://www.cpubenchmark.net/compare.php?cmp[]=2275&cmp[]=2341&cmp[]=2226
Don't get fooled by the "server" designation, the E3 Xeons are basically rebranded i7 with optional (!) ECC support. They do not require any "specific setup", and any decent mainboard manufacturer has them in their list of supported CPUs. Or just go for the non-k i7.
Nothing against the Seasonic models. They're good PSUs in general, certainly no firecrackers, and 80+ is decent.
But decent isn't good.
Efficiency is more important than you might think, besides the implied assumption of higher quality components, a not so efficient PSU will easily turn 30-50W into nothing but heat.
For grandpa John Doe who uses his PC one hour a week it won't make a difference, but if you use it more regularly, your few bucks worth of "saving" will vanish in a couple of months.
Personally i would recommend some Enermax Platimax in that price range, sadly though they're twice as expensive in the US. So, maybe take a look at the Enermax Revolution xt - pretty much same price as your Seasonic, but Gold standard. And there's of course alternatives, like the Enermax DigiFanless, Seasonic Platinum series, BeQuiet Straight / Dark Power, and so on...
You're not "losing" onboard graphics. Any cheap 50$ mobo has a passthrough for integrated CPU graphics. Modern Intel HD are also capable of 2x 1080P.
But let's assume you want
a dedicated GPU, why don't you just go for some cheaper GT 720/30, which comes at half the price, has at least twice the video memory, way better performance, and comes passive cooled.
The Quadro NVS doesn't make any sense if you're not going for professional graphics work, or plain bragging rights.
SSDs are way more relieable than HDDs, in every regard.
M2 cards can be faster due the interface, doesn't mean they always are, or it's necessarily noticeable outside of benchmarks. Matter of slot availability and usage.
If you want to go for benchmark bragging rights, or actually need that bandwith, then have a look at PCIe flash memory. One can also go nuts with software / hardware ramdisks.
If you got a lot of random reads / writes, and don't need several TB of space, then SSDs are the way to go.
The mentioned CPU coolers are still on the market, and should be decided on if you OC or not. Example: the Noctua D15 can easily keep up with more pricy water cooling solutions, easily surpasses them in a noise to cooling ratio, but would be complete overkill if you never OC heavily. Sidenote: keep in mind that coolers have different memory height clearance.
And as i told you from the start: it all depends on the usage.
(and a small edit: i'm not sure why you think about any OC stuff in the first place)
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